1. Field of the Invention
The present invention relates to measurement of heart motion and fast dynamic processes in the body employing harmonic phase (HARP) magnetic resonance imaging and, more specifically, permits such imaging in both two dimensions and three dimensions including measurement of Eulerian strain and Lagrangian strain.
2. Description of the Prior Art
Numerous technologies have been known for use in magnetic resonance imaging. Included in these are magnetic resonance tagging, phase contrast, magnetic resonance imaging and echo-planar magnetic resonance imaging.
In connection with magnetic resonance tagging, the generation and use of SPAMM tag patterns has been known. (See Axel et al., Heart wall motion: improved method of spatial modulation of magnetization for MR imaging, Radiology, 172:349-350, 1989). SPAMM techniques involve generation of an amplitude modulation of underlying anatomy that gives rise to harmonic peaks.
It has also been known to use complimentary SPAMM (CSPAMM) as a tagging method. (See, generally, Fischer et al., Improved myocardial tagging contrast, Mag. Res. Med., 30:191-200, 1993.)
It has also been known to attempt to rapidly acquire images of tagged patterns in motion as by the breath-hold imaging method. (See, Atalar et al., Minimization of dead-periods in MRI pulse sequences for imaging oblique planes, Mag. Res. in Medicine, 32(6):773-777, December 1994.) This prior art approach discloses a gradient-echo, multi-shot EPI (echo planar imaging) acquisition of a sequence of tagged images in a single slice within a breath-hold.
It has also been known to employ phase contrast (PC) magnetic resonance imaging as a means to directly measure motion by measuring a property sensitive to velocity. (See, generally, Pelc et al., Optimized encoding for phase contrast flow measurement, In Proc. Soc. Mag. Res. in Medicine, page 475, Soc., Mag., Res. Medicine, 1990, Annual Meeting, abstract only, and Wedeen, Magnetic resonance imaging of myocardial kinematics technique to detect, localize, and quantify the strain rates of the active human myocardium, Mag. Res. Med., 27:52-67, 1992.) Sequences of velocity fields or strain rates can be reconstructed and by integration the tracking of material points through the sequence and monitoring of the evolution of strain rate can occur. A significant distinction between tagging methods and PC methods is that PC encodes displacement in the transverse magnetization while tagging encodes it in the longitudinal magnetization. As a result, PC can generally image only small motions. PC also tends to yield very noisy images which have a low signal to noise ratio and is not suitable for fast imaging.
It has also been known to employ DENSE as disclosed in the PC literature. (See, generally, Aletras et al., High resolution strain analysis of the human heart with fast-DENSE, J. Magn. Res., 140:41-57, 1999.) It is described as a stimulated echo technique, but also can be interpreted as a tagging technique.
It has been known to use multi-shot EPI acquisition of tagged images. (See, generally, Atalar et al., Minimization of dead-periods in MRI pulse sequences for imaging oblique planes, Mag. Res. in Med., 32(6):773-777, 1994.)
Limiting the field-of-view in order to speed up magnetic resonance image acquisition has been suggested in two new technologies, i.e., SMASH (see, generally, Sodickson, et al., SMASH real-time cardiac MR imaging at echocardiographic frame rates, In Proc. of the Seventh Meeting of the International Society for Magnetic Resonance in Medicine, Philadelphia, Pa., May 1999, abstract 387) and SENSE (see, generally, Weiger et al., High performance cardiac real-time imaging using SENSE, In Proc. of the Seventh Meeting of the International Society for Magnetic Resonance in Medicine, Philadelphia, Pa., May 1999, abstract 385). Neither of these methods has been directly applied to tagged images.
Major developments over the past decade in tagged cardiac magnetic resonance (MR) imaging have made it possible to measure the detailed strain patterns of the myocardium in the in vivo heart. MR tagging uses a special pulse sequence to spatially modulate the longitudinal magnetization of the subject to create temporary features called tags, in the myocardium.
Fast spoiled gradient echo imaging techniques may be employed to create CINE sequences that show the motion of both the anatomy of the heart and the tag features that move with the heart. Analysis of the motion of the tag features in many images taken from different orientations and at different times can be used to track material points in three dimensions, leading to detailed maps of the strain patterns within the myocardium.
Tagged MRI has figured prominently in a many recent medical research and scientific investigations. It has been used to develop and refine models of normal and abnormal myocardial motion, to better understand the correlation of coronary artery disease with myocardial motion abnormalities, to analyze cardiac activation patterns using pacemakers, to understand the effects of treatment after myocardial infarction, and in combination with stress testing for the early detection of myocardial ischemia.
Fast, automated, and accurate analysis of tagged images using the harmonic phase (HARP) process has recently been described. Single-shot HARP (SHARP) has been described to process tagged images in the computation of Eulerian strain. Cine HARP (CHARP) has been described to track motion in image sequences and to enable the calculation of Lagrangian strain. Both methods have utility in the diagnosis of myocardial health.
In spite of the foregoing prior art systems, there remains a need for an improved system for accurate, rapid imaging of heart motion and other dynamic process in the body.
PCT Application Ser. No. PCT/US00/10232, filed Apr. 14, 2000 (based upon Provisional Application No. 60/130,595, filed Apr. 22, 1999) disclosed cardiac motion tracking employing harmonic phase images acquired using magnetic resonance imaging in order to track material points and calculate Lagrangian strain in the heart. Prior U.S. patent application Ser. No. 09/131,589, filed Oct. 10, 1998 disclosed methods of employing tagged magnetic resonance imaging and associated xe2x80x9cangle imagesxe2x80x9d for determining two-dimensional or three-dimensional strain, small displacements, creating synthetic tag lines and optical flow processing.
The above-described need has been met by the present invention.
This invention aids in the measurement of both heart motion and fast dynamic processes in the body using magnetic resonance (MR) imaging. The harmonic phase (HARP) process is a method of rapidly processing tagged MR images to compute tag lines, optical flow, path lines, and both Eulerian and Lagrangian strain within the heart muscle. This invention extends and improves the capabilities of the HARP methodology through MR scanner modifications that will enable real-time imaging of strain within a cross-section of the heart, full three-dimensional imaging of strain in a single breath-hold, and real-time imaging of dynamical processes within the body using HARP""s bandpass filtering methodology. Real-time imaging of myocardial strain enables diagnosis without breath-holds and permits immediate feedback about cardiac status such as, during stress testing, for example. This will allow patients that are more critically ill to be tested safely.
The present invention employs special modifications to the MR scanner image acquisition protocol to improve and extend the capabilities of HARP and create an entirely new way to image dynamic processes within the human body. The new capabilities include (1) real-time strain imaging with no breath-holds, (2) near real-time imaging of three-dimensional strain within an image plane, (3) three-dimensional imaging of strain over the myocardial volume within a single breath-hold, and (4) real-time imaging of arbitrary dynamic processes within the body using the HARP methodology.
These new capabilities provide several new avenues of clinical care. With imaging of strain with no breath-holds, patients that are incapable of breath-holding will be able to undergo cardiac strain analysis in the MR scanner, as well as other imaging. This was previously not known. Real-time strain imaging with the MR scanner will make it safer to administer pharmaceuticals that create artificial cardiac stress. Full 3-D (three-dimensional) strain analysis within a breath-hold will make a full assessment of myocardial motion abnormalities possible within a stress test protocol. Also, this approach to the real-time imaging of dynamic processes using HARP is different from other existing approaches and should provide faster temporal resolution with fewer artifacts.
All of these techniques taken together make it possible for the MR tagging motion estimation methodology to be practiced in other organ systems and parts of the human body besides the heart.
It is an object of the present invention to provide a method of rapidly measuring heart motion and other fast dynamic processes in the body employing harmonic phase (HARP) methods in an accurate and efficient manner.
It is a further object of the present invention to provide such methods which may be employed with improved tagged magnetic resonance imaging techniques.
It is a further object of the present invention to provide such methods which are usable in both two-dimensional and three-dimensional measurement.
It is another object of the present invention to provide real-time imaging of the heart and other parts of the body.
It is another object of the present invention to select tagging protocols which minimize undesired interference from DC spectral peaks.
It is a further object of the present invention to provide improved means for automatically imaging three-dimensional strain in the heart and other body tissues.
It is a further object of the present invention to provide efficient means of imaging strain with either a single breath-hold or no breath-holds.
These and other objects of the invention will be more fully understood from the following description of the invention with reference to the drawings appended hereto.